The $12-$kg slender rod is attached to a spring, which has an unstretched length of $2m.$ Determine the spring elastic potential energy when $=90$

a$.,15\mathrm{.}7Nm$

b$.,13\mathrm{.}7Nm$

c$.0$

d$.,10\mathrm{.}6Nm$

e$.$ not in the mentioned